1. Technical Field of the Invention
The present invention relates to optical zoom systems, and in particular to optical zoom systems that are compact enough to be used on camera modules designed for mobile telephone handsets and similar devices.
2. Description of Related Art
Cameras modules for installation in mobile devices (e.g., mobile phone handsets, Portable Digital Assistants (PDAs) and laptop computers) have to be miniaturized further than those used on compact digital still cameras. They also have to meet more stringent environmental specifications and suffer from severe cost pressure. Optical zoom camera modules are in general costly, large and more delicate that their fixed focus and auto-focus counterparts. As such, optical zoom camera modules tend not be used on these kinds of mobile devices, and particularly not on the cheaper or smaller mobile devices.
A zoom lens is essentially a lens which can be changed in focal length continuously without losing focus. A standard compact zoom camera module would typically consist of three groups of lenses, two of which are able to move with respect to the other. In such a camera, the change of focal length is provided by moving the variator group (generally the middle group of lenses) and the focus is held by changing the position of the compensator group with respect to both the variator group and the image plane. As the variator group moves from the front to the back of the lens, the other moving lens (the compensator group) moves forward and then backward in a parabolic arc to keep the image focused on the image plane. In doing so, the overall angular magnification of the system varies, changing the effective focal length of the complete zoom lens.
The position of the variator with respect to the rest of the system in standard 35 mm cameras can be dictated by a mechanical cam, and in compact digital still cameras by digital control of encoded stepper motors or similar. In both these cases the positional accuracy of the compensator to the variator, and to the image plane, is critical. On miniaturization for use in mobile devices, the further cost of achieving the combined accuracy of the optics, mechanics, actuator and control loop at this scale becomes prohibitive.
It would be desirable to remove or alleviate the accuracy constraints on actuation and control and reduce the requirement for accuracy on the remaining mechanical parts. This would allow for reduction in cost and therefore allow further size reduction.